Ok, I'll be totally honest. If there was a manga/anime where these 3 summoned each other's demi-servant counterpart, I would totally read/watch that. Shirou would 100% be dominated by Ishtar. Honestly, I think Rin and Parvati would work out well. Sakura pretty much hit her personal jackpot.
Shirou/Parvati, Rin/Muramasa and Sakura/Ishtar would be interesting too. I think Rin would die of cringe if she got Ishtar as her servant. I can't see Shirou and Sakura minding getting their counterparts. If Illya summoned Prism Illya, I'm not so sure how that would go. They get along well enough in F/GO. But I think Sitonai and Illya would probably kill each other.
Another idea I just thought of is: Shirou summoning Chloe, Miyu, Emiya (assassin) or EMIYA (Alter). Him summoning Lancer Artoria already a thing. Rin summoning Best Rin-face or Space Ishtar. Sakura summoning any of the Sakura 5, BB or Kama. Actually, I would really love to see how Shirou would handle Kama.
If Sakura summoned Muramasa, Shinji's reaction would be... interesting, I'm sure.
lmao, cheers mate I didn't even think about Seaweed's reaction! Muramasa would most likely kick his ass if he mistreated Sakura or acted snobbish towards him.
